We know that mantle convection is driven by the heat of Earth’s interior, but what controls Earth’s temperature? The current understanding is that the mantle itself acts as Earth’s primary “temperature regulator,” and its actions depend on the atomic-scale properties of mantle minerals that determine viscosity. The effective viscosity of the mantle depends on the rate at which the mineral grains can deform in response to an applied stress, which in turn is strongly dependent on temperature. Laboratory data indicate that for a given stress a 100°C temperature increase lowers the viscosity by about a factor of 10. Consequently, if Earth were to heat up, it would convect more vigorously and lose heat faster. As heat is lost, temperature drops and convection slows, decreasing the rate of heat loss. This temperature-viscosity feedback should keep Earth’s internal temperature well regulated. The temperature at which the thermostat is most likely to be set is just below the melting point of mantle rock because there is an even faster decrease in viscosity with temperature once the mantle begins to melt.
The temperature-viscosity feedback model is useful but it implies a steady system that undergoes only slow changes over long periods of time. This implication is at odds with much of what we know and suspect about mantle materials and geological history. For example, the continents, which are an end product of Earth’s evolution, show evidence of rapid growth spurts (Question 5), which may or may not be associated with accelerated plate tectonics (Hoffman and Bowring, 1984). The seafloor of the western Pacific Ocean contains enormous volcanic mountain ranges, which suggests that the Cretaceous Period (65 to 150 Ma) was a time of exceptionally intense volcanic activity and possibly also fast seafloor spreading (Engebretson et al., 1992). We also know that the Cretaceous was a period of exceptional global warmth and high sea level (Question 7) and stability of Earth’s magnetic field. These observations as well as theoretical considerations raise the question of whether Earth’s thermal evolution and internal processes are adequately described by our (quasi-) steady state models or whether the evolution has been unsteady and punctuated by catastrophic reconfigurations. Thus, even though we understand the most basic features of mantle convection, our level of understanding is insufficient to explain many of the most important geological and geochemical features of our planet.
We are even further from understanding the internal evolution of other rocky bodies of our Solar System, where we have fewer data, and interactions between thermal evolution and orbital evolution provide additional complications (see Box 2.1). Earth (and possibly Venus) has apparently maintained a high enough internal temperature to ensure continued geological activity. However, on smaller planetary bodies, geological surface activity has either long since stopped (Moon) or slowed greatly (Mars). It is believed that the mantles of other terrestrial planets should function in the same way as Earth’s, unless there are different amounts of radioactive elements or different amounts of water dissolved in the mantle minerals. The addition of tiny amounts of water to mantle minerals would lower both the viscosity of the mantle and the melting temperature (Question 6) and may prolong a planet’s geologically active life.
The viscosity of Earth’s mantle is sufficiently low and sensitive to temperature that convection can include complex small-scale currents. Evidence of this small-scale convection is provided by hot spots—large clusters of volcanoes, the most active of which are in Hawaii, Iceland, the Galapagos Islands, Yellowstone, and Réunion (Indian Ocean). Hot spots are usually explained as the surface outpourings of magma formed in mantle plumes, which are cylindrical upwellings of hot (and hence low viscosity) rock that are thought to form near the base of the mantle and rise to the surface at rates much faster than plate velocities (Figure 2.2). Mantle plumes should form as a consequence of heat entering the bottom of the mantle from the much hotter outer core.
Mantle plumes may also be responsible for large igneous provinces, which are vast basalt lava plateaus on continents and the ocean floor. The best current explanation is that they form when the bulbous top of a new plume approaches Earth’s surface (Figure 2.2), then spreads out and causes widespread melting (Ernst et al., 2005). These large, rapid lava outpourings may have caused major perturbations to Earth’s climate (Question 7)